Various abbreviations that appear in the specification and/or in the drawing figures are defined as follows:
3GPPthird generation partnership projectAI/AICHacquisition indicator/acquisition indicator channelCAZAC constant amplitude zero autocorrelation (e.g., Zadoff-Chu sequence)DLdownlinkEUTRANevolved UTRAN (also referred to as LTE or 3.9G)GSMglobal system for mobile telecommunicationsLTElong term evolutionMMEmobility management entityNode Bbase stationPCpower controlPRACH physical random access channelRACHrandom access channelUEuser equipmentULuplinkUTRANuniversal terrestrial radio access networkWCDMAwideband code division multiple access
The following 3GPP specifications are relevant to these teachings:                TS 25.211, v8.4.0 (2009-03) Physical channels and mapping of transport channels onto physical channels (RACH at section 4.1.2.4; PRACH at section 5.2.2.1; and AICH at section 5.3.3.7);        TS 25.214, v8.5.0 (2009-03) Physical layer procedures (PRACH at section 5.1.1; and physical random access procedures at section 6.1);        TS 25.331, V8.2.0 (2008-03) Radio Resource Control (PRACH selection at section 8.5.17).Similar teachings for E-UTRA are in technical specifications (TS) bearing a leading 36 series number.        
As an overview and example of network access procedures for the WCDMA system, reference is made to FIGS. 1A and 1B, of which FIG. 1B is reproduced from FIG. 4 of 3GPP TS 25.211 (v 8.0.0) at section 5.2.2.1.1. Briefly, the UE seeking access to the system transmits on a RACH a first preamble to an access node (base station) and listens on an acquisition indicator channel AICH for a corresponding acquisition indicator AI. If that AI is not received, the UE tries again by transmitting a second preamble and again listens for a corresponding AI. In the example, this continues a third time, at which FIG. 1A shows that an AI corresponding to the third preamble is received by the UE. Only after receiving that AI does the UE then transmit what is termed the message part of its access message. The preambles and message parts are shown also at FIG. 1B. Generally, each of those three preambles uses a signature sequence that the UE randomly generates for each of the different preambles. This avoids different UEs attempting access over the same RACH from reading the AI from the other UE, since the AI is mapped to the signature sequence used in the preamble to which the AI corresponds. While the example shows three preambles, this is not to be seen limiting, and there can be one up to a network-configured maximum number of preambles, depending on channel conditions.
In general, the transmit power that the UE uses on the PRACH for the first preamble is set by higher layers. As currently set forth in 3GPP TS 25.214 at section 6.1, the UE randomly selects a signature set (e.g., a CAZAC sequence for LTE-specific implementations) and sets the transmit power for its initial access attempt on the PRACH to a commanded preamble power set by higher layers (shown as Pinit at FIG. 2). If the UE finds no positive or negative acquisition indicator on the AICH that corresponds to the slot at which the UE sent its first preamble on the PRACH at power Pinit, the UE then randomly selects another signature sequence and as shown at FIG. 2 re-attempts access using an increased power, where the increase is given as a power ramp step P0. The UE continues these transmissions, repeatedly using a different sequence and increasing transmit power by the ramp step until either it finds the acquisition indicator at the corresponding slot of the AICH or its counter hits a maximum, which may or may not be three as in this example. Once the UE receives its acquisition indicator on the AICH, then as can be seen at FIG. 2 the UE can send the message part of the PRACH.
There are several problems seen with the above approach. As seen at FIG. 2, when the first preamble does not result in the acquisition indicator being received at the UE, the UE must repeat the process with new signature sequence and increased power Pinit+P0. If there is a reception problem at the BS, or if there is interference in the first-sent preamble, then the second-sent preamble represents needless power consumption at the UE, which is a consideration with portable electronic devices. Also with the second-sent preamble there is an added computational complexity for the UE which needs to map the randomly generated sequence used in each preamble to the corresponding slot in the AICH at which the UE looks for its acquisition indicator. These teachings are directed toward addressing at least some of the above concerns.